The invention relates to a process for identifying disturbance from other light sources when taking readings from a linear optical sensor of a goniometer, of a type used for measuring angles of wheels of motor vehicles. In this kind of sensor, the light originates in a beam aimed in a predetermined direction, from which an angle to be measured can be determinedxe2x80x94the angle between the direction of the beam and an axis of reference of the goniometer which is perpendicular to the linear optical sensor. The sensor is usually constituted by a plurality of contiguous and aligned photo-sensitive elements, through which an xe2x80x9cimagexe2x80x9d of the light source can be built up from the ordered set of incident radiation intensities registered on the photo-sensitive elements. The measurement of the angle is a direct function of the distance of the image from the measurement origin in a longitudinal direction, the origin being defined by the intersection of the reference axis with the longitudinal axis of the sensor. Therefore by measuring this distance, on the basis of the geometrical characteristics of the system, it is easy to measure the angle with a good degree of precision.
It is obvious that a correct measurement depends on the ability of the receiving device (comprising the sensor) to identify the xe2x80x9crightxe2x80x9d light signal, i.e. the light beam used to measure the angle, distinguishing it from other light signals (disturbance) which are inevitably present in the environment in which the goniometers forming the apparatus for measuring angles of motor vehicle wheels operate.
In known apparatus immunity from this disturbancexe2x80x94i.e. the ability to distinguish the right source beam in order to measure the wheel angle from all the other beams in the environment, is achieved using a process in which at least two xe2x80x9cimagesxe2x80x9d are received by the receiver at two different times, characterised by two different conditions of the source producing the beam which gives rise to the signal used for measuring. In other words a reading is taken when the source is lit (so the measurement by the sensor is certain) and another measurement when the source is off (thus the correct beam is not received and is not present in the corresponding xe2x80x9cimagexe2x80x9d). In order for this to be possible it is necessary to synchronise the state of the light source (on or off) with the reading of the two corresponding images. The identification of the image of the light signal usable for effecting the measurement by the sensor is done by a control unit which carries out a comparison process by subtraction of the two xe2x80x9cimagesxe2x80x9d acquired in the above-described conditions. The main limitation of the prior art realisations consists in the fact that it is necessary to establish a connection of some kind between the source and the receiver in order to be sure that the single acquired images are unmistakeably associated. This calls for the use of special means which lead to a certain constructional complication in the apparatus for measuring the angles which are characteristic of the wheels of the vehicles.
Other methods for excluding the disturbance in the surrounding environment, though limited in number, combine light sources with special light-emittance characteristics with optic filters positioned in front of the sensor. These devices have the drawback of being able to function correctly only where the environmental disturbance does not include characteristics which are identical or similar to those of the light emitted by the source.
The main aim of the present invention is to obviate the limitations and lacks in the prior art.
An advantage of the invention is its overall simplicity.
These aims and advantages and others besides are all achieved by the present invention, as it is characterised in the claims that follow.
In the process of the invention a light signal is produced by a light beam having a predetermined direction with respect to which an angle to be measured is measured, the angle being formed between the direction of the light beam and a reference axis of the goniometer which reference axis is perpendicular to the linear optical sensor. The linear optical sensor is constituted by a plurality of contiguous and aligned photo-sensitive elements, by means of which elements an image of the light signal can be read. The image is constituted by an ordered totality of intensities of incident radiations recorded on the photo-sensitive elements. Measurement of the angle is a direct function of a distance of the image from an origin for measurement of distances in longitudinal direction, which is defined by an intersection of the reference axis with a longitudinal axis of the sensor.